Weight back is a good way to go, but I try to let the front wheel do its own thing a little bit... Instead of strictly adhering to the line you have chosen, it is better to gently coax your bike in the direction you want to go. I agree with Darrin, I typically attack sand like I would a hill, and imagine myself trying to get on a plane (like boating). I'm 220 riding a 26'er, and I don't have too much trouble in sand. Just like anything else in mountain biking, practice makes perfect!